In a conventional electrophotographic process, high voltage (5,000-8,000 volt DC) is applied to a metal wire to produce a corona discharge, which, in turn, causes a photo-conductive drum to become charged. In this conventional process, the metal wire is the "charging" component, and the photo-conductive drum is the charging receiving or "charged" component. Because of the high voltage involved in the conventional charging process, undesirable side products such as O.sub.3 and NO.sub.x are often produced during the corona discharge which could degenerate the surface of the photoconductive drum. When the surface of the photo-conductive drum becomes degenerated, the print image produced from the electrophotographic process becomes blurred and its quality deteriorated. Also, the surface of the metal wire used in the conventional electrophotographic process often becomes contaminated with impurities which could also result in deteriorated quality in the print images. As the trend in the electrophotographic process is to use photo-conductive drums containing organic photo-conductive material, the organic photo-conductive material can easily react with some of the reactive materials generated during the corona discharge. This also causes the image quality to degrade. Another disadvantage of the conventional electrophotographic process is that most of electrical current is lost to the shield grid behind the photo-conductive drum, only 5-30% of the current from the corona discharge is received by the photo-conductive drum, thus resulting in a relatively low charging efficiency.
In Jpn. Kokai Tokyo Koho JP.58-150975 (the '975 reference), it is disclosed a direct contact type charging component which causes a charged component, which is a photo-conductive drum, to become charged via direct contact therebetween. In the '975 reference, the charging component is a cylindrical roller which comprises a metal core enclosed by an electrically conductive rubber material containing electrically conductive carbon black particles which arc dispersed within the rubber matrix. The direct contact charging component design of the '975 reference was intended to avoid the aforementioned problems associated with the conventional charging component using the corona discharge method; however, it also suffered from other problems. Most notably, a significant portion of the rubber matrix becomes abraded due to frictional loss resulting from constant direct contact between the charging component and the charged component. When the extent of abrasion exceeds a certain level, some of the carbon black particles would become protruded and cause the photoconductive drum to be scratched and thus damaged. A damaged surface of the photo-conductive drum would produce defective images such as striation.
Several polymeric materials have been used to form a protective surface layer for the direct contact type charging components. Jpn. Kokai Tokyo Koho Jp.50-13661 ('661 reference) discloses a charging component which comprises a charging roller enclosed by a surface layer made of polyamide or polyurethane. Such a protective surface layer minimizes some of the abrasion problems of the direct contact charging component disclosed in the '975 reference. However, the polyamide and polyurethane surface layer was found to be environmentally unstable. In particular, the volume resistivity of the polyamide and polyurethane surface layer at low temperature and low humidity conditions can be about three orders of magnitude greater than that at normal conditions. The charging capacity of a charging component substantially decreases if the volume resistivity thereof is too high. The high volume resistivity of a charging component could also result in non-uniform charging. In an electrophotographic process utilizing the discharged area development (DAD) technique, the non-uniform charging causes spotty images to be generated.
The high volume resistivity of the surface layer also necessitates a high-voltage charging condition. Upon direct contact between the charging component and the charged component, any defect in the charged component could cause a discharge breakdown, resulting in a non-uniform charge density on the surface of the charged component. During electrostatic charging, the point of defect becomes the point of leakage by which the point of discharge breakdown serves as a electrostatic sink into which excess current flows from the charging component. Such a current drainage results in a lowered electric potential elsewhere, and, consequently, a region of deficient charging is developed in the charged component. When this occurs, a white band will appear if the print image is produced using the charged area development process. On the other hand, a back band will appear if the discharged area development process is used to produce the print image. Another disadvantage of the surface layer disclosed in the '661 reference is that the high hardness of the Nylon or polyurethane-based surface layer on the charging component could cause damages to the photo-conductive drum.
In Jpn. Kokai Tokyo Koho Jp. 1-205180 (the '180 reference) a charging component containing a surface layer containing N-alkoxyl-methylated Nylon is disclosed. The manufacturing of the N-alkoxyl-methylated Nylon involves a relatively complex process and the cost of manufacturing thereof is high. Furthermore, because of the high hardness of the N-alkoxyl-methylated Nylon, a lining layer comprising a relatively soft rubber material is often required underneath the surface layer to prevent damage to the photo-conductive drum. This further adds to the cost of using the N-alkoxyl-methylated Nylon disclosed in the '180 reference.